Space Tethers

Bothell, Wash., Company Tests Technologies For Catapulting Objects In Space

By Ben Raker

It's only science fiction until you do it.

Those are words to live by for Rob Hoyt, president of Tethers Unlimited, Inc., a ten-person technology company based in Bothell, Wash.

It's an appropriate motto for a researcher developing technologies that are still years off, with many challenges left to solve, but also have the potential to transform how things are moved around in space.

Hoyt was a keynote speaker at the 6th annual meeting of the NASA Institute for Advanced Concepts (NIAC) held Oct. 19-20, 2004, in Seattle. The NIAC focuses on visionary concepts aimed decades into the future. Hoyt, a past recipient of NIAC funding now working under a variety of grants from NASA and other organizations, spoke about lessons his company has learned through its work on space tethers.

"A space tether is essentially a long, thin cable or wire deployed in space," says Hoyt. It's a blanket term that can refer to anything from a cable to hold a wrench on a space walk to the several-hundred-kilometer-long catapult-like devices that scientists are proposing for slinging objects around space.

Tethers are already used on a small scale for restraining objects, but their greater, still-untested potential is to move things around;ndash;and to do so in a way that they can be used many times, without using fuel propellants.

"There are some that think that this is too ambitious a step for tethers, and you know they may be right, I can't say that," says Kirk Sorensen, an aerospace engineer with NASA's In-Space Propulsion Technologies Program. "But there are some that think that, no, we're going after what makes sense."

The transport type of space tether would be designed to orbit the Earth, twirling within that orbit like a baton. Using this spinning motion, tethers could be used to catch and throw various types of objects in space. They might eventually be used to send equipment or vehicles into higher orbits or into the orbits of other planets. Tethers could also have applications for cleaning up clutter in Earth's orbit by capturing it or flinging it down into the upper atmosphere where it would burn up;ndash;Hoyt calls a concept he is developing in this area a "terminator tether."

The technology for accomplishing such goals is still 10 to 40 years away from use, and has skeptics who think these applications would best be left in the world of science fiction. But the potential that tethers have, along with recent experimental successes by Hoyt and others, has kept NASA interested.

"This is a tough problem," says Sorensen. "I don't want to paint a pretty picture here, but the payoff is huge and I think it's a great thing that NASA is having the courage to take a chance on something that's very ambitious but could pay off in a really big way."

One advantage space tethers have over conventional means of moving things in space is that tethers could theoretically be run without consuming propellant. Also, provided that the tether could survive over time, it could be used over and over again, whereas a rocket can be used only until its fuel runs out.

The proposed tethers that would catch and throw other objects into higher orbits are called "momentum exchange" tethers. Once such a tether transferred its energy to the orbit of another object, its own orbit would decline. In order to regain that orbital energy, researchers are looking at ways to have tethers use free electrons found in space in a process known as "electrodynamic reboost."

Especially promising are tethers that would combine the processes of slinging payloads and then recovering: These are known as Momentum Exchange-Electrodynamic Reboost or MXER ("mixer") tethers.

The electrodynamic part of the process would be supported by the Earth's ionosphere. "We think of space as a vacuum, but it's not really," says Sorensen. "There's a very, very small amount of material there and some of it has been ionized, which means the electrons have been stripped off the atoms."

Since a tether is a long, thin object, it can be made to act as a conducting wire for these free electrons. The electrons could be pushed through the wire with energy from attached solar panels. Pushing the electrons from the ionosphere through the wire in the presence of Earth's magnetic field would create a force that would boost the tether back into a higher orbit.

"It sounds really complicated and really fancy, but it's actually the exact same physics that makes every motor on Earth work," says Sorensen. "A motor is literally just a coil of wire in a magnetic field. In this case, you're imagining it on a much bigger scale where the wire is the tether and the magnetic field is the Earth's magnetic field, but in principle, it's exactly the same idea as a motor."

The largest MXER tethers could eventually reach lengths equivalent to more than 500 of Seattle's Space Needles strung end to end;ndash;a prospect Hoyt acknowledges as a long way away. He plans to take things one step at a time, testing small devices and using them for potential commercial applications like cleaning space junk before graduating to more ambitious projects that move big payloads.

"When you're talking about building a twenty-ton, hundred-kilometer long tether system in space, that's quite a challenging endeavor," he said in his talk. "We want to learn those lessons on a smaller scale first."

These modest experiments can still be exciting. In September 2004, Tethers Unlimited conducted tests of their deployment systems inside an airplane flying a parabola pattern to simulate the zero-gravity conditions of space.

"We needed to demonstrate that we could get these devices to deploy and work reliably in a zero-G environment," says Hoyt. This mission focused on testing a prototype that would expand as it deployed in order to capture space junk;ndash;a mechanism that looks like Spiderman casting a web. This experiment was a success, with three nearly perfect deployments.

Through experiments such as this, Tethers Unlimited is gearing up for its first in-space experiment currently scheduled for December 2005. This will be an experiment aimed at one of the major challenges facing space tether progress: making a tether that can withstand the rigors of space, including bombardment by radiation, micrometeorites, and orbital debris.

One of Hoyt's concepts, called the Hoytether, is to build a sturdy tether system that could stretch out in space and consist of a number of redundant, fibrous strands so the tether would not lose overall integrity if one strand were damaged by debris.

Besides making tethers stronger, those in the small community of space tether researchers acknowledge that there are a number of technical challenges that need to be addressed before something like a MXER system can be fielded.

"There are probably not a lot of physics questions about whether the tether will work," says Sorensen. "But that being said, there are a number of engineering challenges as to the feasibility of the tether."

One of the biggest challenges, he says, is developing the tools in order to accurately simulate how the tether will rendezvous with a payload in orbit.

Also, because the massive MXER tethers may cost a lot to get into orbit in the first place, they won't overcome the costs of conventional rocket systems unless they can be reused;ndash;again a reason they must be strong.

"You wouldn't want to build it for just one shot," says Sorenson. "It's meant for a number of payloads."

Still, over time, a successful MXER tether that consistently relayed its payloads to higher orbits or other planets, would be worth its cost and potentially competitive down the line.

"The main competition is the status quo: existing technology, chemical rockets, and now some of the electric propulsion thrusters which have already flown and have been mostly successful," says Hoyt.

Another challenge to overcome is perception. The concept of space tethers is not new; it has been discussed since the early 20th century and several in-space experiments have already been conducted. Some of these have been successful, but two high-profile experiments where electrodynamic tethers broke in space in the 1990s, gave them something of a bad name.

That said, proponents are cautiously optimistic that tethers can be made stronger in the future through work such as that of Tethers Unlimited.

"I've probably never been more optimistic about the success of this technology than I am now, from a technical perspective," says Sorensen, but he adds, "I don't want to negate the fact that there are serious challenges."

For his part, Hoyt is well aware of the risks, but sees promise in developing incrementally more ambitious tests and applications.

At his talk, he encouraged researchers to keep pushing the boundary between science and science fiction for their high-risk, high-payoff ideas.

"They're going to remain science fiction unless you personally go out and bust your hump to make it happen," he said. "The flip side to that is that we've got a wonderful opportunity. You have the opportunity to take a science fiction idea and to turn it into reality."

Ben Raker is a freelance writer and editor who studied science writing at the University of Washington.

For more information about the work of Tethers Unlimited, visit:

www.tethers.com

For information about the NASA Institute for Advanced Concepts, visit:

www.niac.usra.edu

For animations of MXER space tethers, visit:

http://mxer.tntech.edu/animations/topic_view

Images

Top: An artist's rendition of a momentum exchange-electrodynamic reboost (MXER) tether. Image: NASA

Bottom: A space tether used to catapult another object into a higher orbit or eventually to another planet would first catch the object and then transfer its orbital energy to that object, losing momentum of its own. The tether's orbital energy might later be recovered through interaction with the Earth's ionosphere. Image: Tethers Unlimited, Inc.